1. Field of the Invention
The present invention is generally related to a method for attaching a first conductor, such as a copper component, to a second conductor, such as a component made of a ferrous alloy, and, more particularly, for welding a stranded copper wire lead to an electrical conductor extending from an electronic device.
2. Description of the Prior Art
Several ways for attaching a conductor, such as a stranded wire lead or a solid wire lead, to a terminal post are well known. One method comprises the steps of stripping a length of insulation from the wire to expose the metal strands, tinning the wire by dipping the metal strands at the stripped end in a suitable flux, immersing the stripped end in molten solder, wrapping the tinned lead around a terminal post to make a secure mechanical connection and applying heat and a flux-core solder to the connection until the joint is completely covered. The joint is then permitted to cool so that the molten solder solidifies. Finally, the joint is cleaned to remove the excess flux by using a solvent solution. Typically the solvent solution is a chlorinated fluorocarbon or a flammable organic.
In certain situations, limited space between the terminals precludes the wrapping of the stranded wire around the terminal to make a secure mechanical connection. In situations when space is not available to wrap the wire around the terminal, a lap joint is sometimes used. Typically the mechanical and electrical suitability of this type of lap joint is very dependent on operator skills and the results can therefore be very unpredictable.
One possible way to overcome the deficiencies of a soldered joint is to use a resistance weld technique. However, as is well known to those skilled in the art, resistance welding of stranded wire to a terminal post is difficult because of the characteristics of the materials involved. For example, the stranded wire is frequently made of tin plated copper. When exposed to high temperatures, the tin plating evaporates and forms blow holes which result in undesirable porosity within the joint. Another serious problem involves the relative thermal and electrical characteristics of copper and ferrous alloys such as Alloy 42 or Kovar. Alloy 42 is 58% iron and 42% nickel while Kovar also comprises 7% cobalt. These alloys differ significantly from that of copper in their physical and electrical properties. More specifically, the relative resistivity of the ferrous alloys, which are typically used for electrical terminals, is higher than that of copper and the thermal conductivity of these ferrous alloys is lower. The ferrous alloy has a higher melting point than copper, but the higher resistance of the alloy causes it to heat in response to an electric current while the copper, because of its low resistivity, does not experience an equivalent temperature rise. Therefore, the ferrous alloy melts and collapses before the copper reaches its melting point. As a result, the high temperature normally used during welding procedures usually melts the ferrous allow with virtually no change to the copper component. Another problem with processes like those described above is that the pressure applied by a welding electrode sometimes unravels the stranded wire during the welding process. This results in some of the strands not being included within the weld joint.
In view of these problems, it would be significantly beneficial if a welding process could be devised to avoid the problems described above and result in a secure joint between copper strands and a lead terminal made of a ferrous alloy.